P.L. Ng

Rheology of Mortar and its Influences on Performance of Self-Consolidating Concrete

A two-part experimental program is presented in this paper. In the first part, four self-consolidating mortar mixes were designed with different compositions of cementitious materials including cement, pulverized fuel ash and condensed silica fume. For each SCM mix composition, the superplasticizer dosage was varied and a total of 30 batches of mortar were produced. For every batch of mortar, the rheology was determined by a rheometer and the workability was measured by the mini slump flow test and the mini V-funnel test. From the test results, the saturation SP dosage of each SCM mix was determined. In the second part, four self-consolidating concrete mixes were produced, each comprising a SCM mix with saturation SP dosage and a fixed coarse aggregate content. The workability, filling and passing abilities and segregation stability were measured by the slump flow, U-box and sieve segregation tests, respectively. It was found that the performance of all the SCC mixes was satisfactory. The test results suggested that using a SCM mix with saturation SP dosage as the mortar phase can produce SCC with high performance and therefore is a good starting point to optimize the performance of SCC mixes.

Adiabatic Temperature Rise of Pulverized Fuel Ash (PFA) Concrete

Owing to the less exothermic pozzolanic reaction of pulverized fuel ash (PFA) compared to cement hydration, the addition of PFA can reduce the heat generation of concrete during its hardening. However, as the water to binder (W/B) ratio would affect the proportions of cement and PFA that could react with water, the conventional practice of determining concrete temperature rise solely based on the cement and PFA contents may not yield accurate estimations. An experimental programme was launched to investigate the adiabatic temperature rise of PFA concrete mixes. Seven concrete mixes without PFA added and 14 concrete mixes with PFA dosages at 20% and 40% were tested with the recently developed semi-adiabatic curing test method. The adiabatic temperature rise was obtained by applying heat loss compensation to the test results. It was found that the incorporation of PFA could suppress the adiabatic temperature rise by 4°C to 14°C. The test results revealed the dependence of adiabatic temperature rise on both PFA dosage and W/B ratio, whose combined effects can be accurately addressed via the prediction formula and design chart developed herein.

Adiabatic Temperature Rise of Condensed Silica Fume (CSF) Concrete

Condensed silica fume (CSF) is often added into concrete mixes to enhance the properties of concrete. However, the effect of CSF on the heat evolution and temperature rise of concrete is not clearly known. Test results in the literature are insufficient and sometimes contradictory to enable any conclusion to be drawn regarding the role of CSF in heat generation behaviour of concrete. Moreover, since the chemical reactions of cement and CSF both involve water and hence cement and CSF are competing with each other in reacting with water, the water to binder (W/B) ratio may affect the temperature rise characteristics of concrete. This paper reports an experimental study of adiabatic temperature rise of CSF concrete conducted at The University of Hong Kong. Five concrete mixes without CSF and 10 concrete mixes with CSF dosages at 5% and 10% were tested with the recently developed semi-adiabatic curing test method. The adiabatic temperature rise was obtained by applying heat loss compensation to the test results. It was found that the addition of CSF could suppress the adiabatic temperature rise of concrete. At the same time, the strength of concrete could be enhanced. Based on the experimental results, prediction formula and design chart of adiabatic temperature rise of CSF concrete were developed.

Adiabatic Temperature Rise of Incompletely Hydrated Cement Concrete

The temperature rise of concrete during hardening is intimately related to the mix proportion, among which the cement content is a major factor. However, high-strength concrete mixes are often proportioned with low water contents which leads to incomplete hydration of cement contained therein. Hence, the conventional rule of determining concrete temperature rise solely based on the cement content may not yield accurate estimations. An experimental program has been launched to investigate the coupled effects of cement and water contents on the adiabatic temperature rise of concrete. Eighteen concrete mixes were tested with a newly developed semi-adiabatic curing test method and their adiabatic temperature rise obtained by applying heat loss compensation to the test results. The results revealed that, when the water/cement ratio is lower than 0.36, both cement and water contents have effects on the adiabatic temperature rise of concrete. Prediction formula and design chart of adiabatic temperature rise, which are accurate to ±1.3°C compared with the test results, are developed. Furthermore, prediction formula of the degree of hydration of concrete is recommended.